Multifilar sublimation filament for getter vacuum pumps



M r 1967 A. B. FRANCIS ETAL MULTIFILAR SUBLIMATION FILAMEN'I FOR (BETTER VACUUM PUMPS Filed Aug. 27. 1964 //A ///Y////:/F/////// INVENTORS ARTHUR B. FRANCIS WESLEY H. HAYWARD United States Patent f 3,311,776 MULTIFILAR SUBLIMATION FILAMENT F012 GETI'ER VACUUM PUMPS Arthur B. Francis, Los Altos Hills, and Wesley H. Hayward, Mountain View, Calif assignors to Varian Associates, Palo Alto, Calif, a corporation of California Filed Aug. 27, 1964, Ser. No. 392,417 3 Claims. (Cl. 313178) This invention relates to vacuum pumps of the gettering type and particularly to novel sublimation means for use in such pumps.

Pumping by thermally evaporating or subliming getter materials onto the internal walls of a pump housing suitably forepumped has been well known for many years. Gas molecules coming into contact therewith combine chemically or physically with the condensed material and are removed from the gaseous state so as to reduce the pressure. Using pumping apparatus of this type, extremely high pumping speeds have been achieved, in excess of 3500 liters/ sec. for the active gases such as 0 N 00 and CO which can be scaled up as the need arises. While such apparatus may not be employed for pumping the noble gases, in combination with an ion or a diffusion vacuum pump, which can pump the noble gases and other inactive gases to some degree, an even more practical and useful pump apparatus results.

In the past, the sublimation means ordinarily comprised a filament made of a metal having a high melting point (e.g., refractory metals such as tungsten 0r tantalum) in the form of a rod, a single strand or a rope, originally in good thermal contact with a coating or coil of a metal having a lower subliming temperature than the melting point of the filament and having good gettering properties (e.g., reactive metals such as titanium or zirconium). The getter was supported by the filament and initially was in good thermal contact therewith throughout its length. A voltage was then applied across the sublimation means. Heating occurred as a result of the current passing therethrough until a temperature was reached at which the getter material was caused to sublime.

When the sublimation means was initially turned on there was current flowing in both the filament and in the getter material. Temperature was usually fairly constant in time yielding a constant sublimation rate. However, it was observed that with passage of time a small region developed (hot spot) along the sublimation means where the temperature was highest. As a result, the evaporation rate from this region was higher than any place else on the sublimation means. The cross-section of getter material decreased and the hot spot region became even hotter in relation to the remainder of the sublimation means. Hence, the evaporation rate increased drastically until eventually there was no getter material left at the hot spot.

Thereafter, current flowed only through the filament, which current was normally too low to produce the needed heating to sublime the remaining getter material. Attempts at increasing current through the filament by increasing the voltage thereacross usually resulted in filament burn-out leaving from 50%80% of the getter material unused.

Accordingly, it is the principal object of the present invention to provide an improved sublimation means for use, for example, in getter vacuum pumps and which is capable of dispensing a higher percentage of getter material.

Briefly stated, in accordance with one teaching of the present invention, there is disclosed a sublimation means comprising a length of refractory filament overwound with a multifilar layer in direct thermal contact there- 3,3 l l ,Tib Patented Mar. 28, 1967 with. The layer includes two or more wires of getter material and a single wire of a refractory metal having a melting point higher than the subliming temperature of the getter material. The filament is preferably a cyclin drical core as of tungsten, having a diameter d. The overlay comprises two, preferably, or more wires of getter material, as of titanium, having a diameter on the order of d or slightly greater than d, and :a single refractory metal wire as of molybdenum having a diameter less than a, preferably of order d/ 2.

One feature of the present invention is the provision of a novel sublimation means comprising aleugth of refractory filament overwound with a multifilar layer in direct thermal contact therewith, said layer comprising two or more Wires of getter material and a single wire of a refractory metal.

Another feature of the present invention is the provision of sublimation means of the above type wherein the filament has a diameter d, the getter material wires have a diameter on the order of d or slightly greater than d and the refractory metal wire has a diameter less than d, preferably d/2.

Still another feature of the present invention is the provision of means of the above type wherein the filament is made of tungsten, the getter material is made of titanium and the refractory metal wire is made of molybdenum.

These and other objects and features of the present invention and a further understanding may be had by referring to the following description and claims, taken conjunction with the following drawing in which:

FIG. 1 is a view partially broken away of a sublimation cartridge unit employing the novel sublimation means of the present invention; and,

FIG. 2 is an enlarged fragmentary view delineated by the arrows 2-2 of FIG. 1.

Referring now to FIG. 1 there is shown a sublimation cartridge unit 19 employing the novel sublimation means 1 1 of the present invention. Unit 10 includes a plurality of sublimation means 1 1 which may be individually energized as each one is used up. Sublimation means 11 are each removably held at one end against metal rod leads 1 2 by set screw clamp means 13 and at the opposite end by a single set screw clamp means 14 supported on rod 15. Loosening and tightening of these screws is all that is necessary to replace a sublimation means 11. An apertured ceramic spacer plate 16 prevents shorting out of sublimation means 11. Support rod 15 and rod leads 1-2 are supported by and passed through, in vacuum tight manner, an all metal vacuum sealing flange 17. The rod leads 12 are electrically isolated from flange 17 by ceramic insulator assemblies 18 welded in vacuum tight manner to rod leads 12 on the one hand and flange 17 on the other rand. Flange 17 is adapted to mate with, in vacuum tight manner, another all metal vacuum sealing flange 19 welded on a walled passage 20 leading into the housing of a vacuum pump 21.

As best seen in FIG. 2, the novel sublimation means 11 includes a length of refractory filament 22 and an overlay 23. Filament 22 is preferably a long core, as of tungsten or tantalum. In a typical embodiment, filament 22 is a tungsten core of 30 mil diameter and 6" long.

Overlay 23 is made up of a plurality of wires 2-4 of getter material and a single wire 25 of a refractory metal as of molybdenum or colombium, helically twisted tightly about the filament 22 so that the overlay 23 is in direct thermal contact with the filament 22 throughout its lengh. In a preferred embodiment the getter material is titanium. However, any one of a number of materials or alloys may serve as the getter material in the sublimation means of this invention. Zirconium, uranium and other metals particularly in Groups 1V and V of the Periodic Table, for example, can be employed. Aluminum, alkaline earth metals and the alkaline metals are still other metals which may be used to advantage. In addition, wires 24 are preferably of a diameter on the order of or slightly greater than the diameter of filament 22.

The exact role of the refractory Wire 25 is not completely understood. However, it does aid in preventing the flow of getter material when it becomes molten. If fiow should occur large droplets form which may alloy with the filament 22 and cause burn-out. The diameter of the wire 25 should be less than the diameter of filament 22 and preferably one-half the diameter of the filament 22.

In a typical embodiment, the overlay 23 is trifil atr being made up of two 31 mil titanium wires and a 15 mil molybdenum Wire.

In operation, the pump 21 and the system to be evacuated (not shown) to which pump 21 is connected are partially evacuated (roughed) down to 10 microns with a mechanical pump or a refrigerated sorption pump (not shown). A power supply (not shown) of, for example, -6 volts is connected to one of the sublimation means 11, and current passed therethrough, for example, 30 amperes, which is insufiicient to cause sublimation of any of the getter material, but effects out-gasing of same. The evolved gas is pumped by the roughing means which then may be isolated from the remainder of the system. The power supply is then run at, for example, 4547 amperes to cause sublimation of the getter material. At much higher currents the getter 24 will become overheaded, melt, alloy with the filament 22 and corrode same, although the means 11 may be run at higher currents for short periods of time without destruction of same.

The diameter of filament 22 may vary considerably. However, the respective dimensions of the getter material wires 24 and refractory wire 25 in relation to the crosssectional dimension of the filament 22 are chosen so as to obtain maximum utilization of available getter material. Effective subliming of getter material occurs at the point and slightly beyond, where the cross-section of the getter material is on the order of the cross-section of the filament 22. However, if the cross-section of the getter material wires 24 is chosen to be much larger than that of the filament 22 the wires are difficult to Wrap around the filament and more current is required to cause effective sublimation of getter material with the result that increased current demand may lead to filament burn out. The dimensions of the refractory wire 25 are chosen with a view towards obtaining maximum density of getter material per unit length while still permitting the refractory wire to effectively perform its function of preventing getter material flow.

A single overlay is to be preferred over a multiple overlay, for example, as multiple overlay devices in general are much more difiicult to produce in mass quantities. Moreover, the single overlay of the present invention is compatible with presently available power supplies in so far as current and voltage requirements are concerned.

For optimum getter material utilization the getter should be sublimed in proportion to the pressure in the system. At high pressures the getter is consumed rapidly and therefore the sublimation means should be operated at duty-cycle. At pressures below 10- torr down to 10 torr approximately 25% duty-cycle is sufiicient to remove gas as fast as it enters the system. When operating the sublimation means periodically care should be taken to see that the initial current is not exceeded if premature burn-out is to be avoided. Sublimation means of the type disclosed herein have been made and have an approximate operating life of 20 hours running at 100% duty-cycle. Moreover, approximately 70% of the available getter material is utilized which is some 40% better than any known prior art device. When operated in combination with ion pumps they have demonstrated useful pumping speeds from 0.1 to 1O torr and even lower.

Since many changes can be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A sublimation means comprising a length of refractory filament having a diameter d and a multifilar layer wound over said filament in direct thermal contact therewith, said layer comprising two or more wires of getter material having a diameter on the order of d and a single wire of a refractory metal having a diameter less than d.

2. The means according to claim 1 wherein said refractory metal has a diameter of approximately d/ 2.

3. A sublimation means comprising a tungsten core and a trifilar layer wound over said core in direct thermal contact therewith, said layer comprising two titanium wires and a molybdenum wire, the cross-sectional dimension of said titanium wires being approximately equal to the cross-sectional dimension of the tungsten core and the cross-sectional dimension of the molybdenum wire being approximately one-half the cross-sectional dimension of the tungsten core.

References Cited by the Examiner UNITED STATES PATENTS 2,731,581 1/1956 Krefit 313344 X 2,804,564 8/ 1957- Couch 3'131'80 2,837,680 6/1958 Leferson 313- 2,960,618 11/1960 Waer 313-180 X 3,152,689 10/1964 Connor 313-7 X 3,240,970 3/1966 =Reifenschweiler 3131'80 X FOREIGN PATENTS 834,655 5/1960 Great Britain.

References Cited by the Applicant UNITED STATES PATENTS 2,796,555 6/1957 Connor. 2,925,214 2/ 1960 Gurewitsch et al. 2,986,326 5/1961 Landfors. 3,117,210 1/1964 Herb. 3,140,820 7/ 1964 Clausing.

JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

3. A SUBLIMATION MEANS COMPRISING A TUNGSTEN CORE AND A TRIFILAR LAYER WOUND OVER SAID CORE IN DIRECT THERMAL CONTACT THEREWITH, SAID LAYER COMPRISING TWO TITANIUM WIRES AND A MOLYBDENUM WIRE, THE CROSS-SECTIONAL DIMENSION OF SAID TITANIUM WIRES BEING APPROXIMATELY EQUAL TO THE CROSS-SECTIONAL DIMENSION OF THE TUNGSTEN CORE AND THE CROSS-SECTIONAL DIMENSION OF THE MOLYBDENUM WIRE BEING APPROXIMATELY ONE-HALF THE CROSS-SECTIONAL DIMENSION OF THE TUNGSTEN CORE. 